Heretofore various sorts of systems have been researched and developed as communication systems. Among them the spread spectrum communication system is known as a system having a high reliability.
By this spread spectrum communication system, on the transmitter side, a primary modulated signal of data in a narrow frequency band of base band information, sound, etc. is transmitted with spread spectrum obtained by hopping to a plurality of frequence in a wide frequency band with a high speed (FH method, frequency hopping), by spreading spectrum in a wide frequency band by using a high spread pseudo noise code (PN code) (DS method, Direct Sequence), or by combining them (FH/DS method), and on the receiver side, an information signal is reproduced by inversely spreading the wide band signal to the original narrow band primary modulated signal by means of a correlator. Recently attention is paid to this spread spectrum communication system as a communication system having a very high reliability from several points of view that it is resistant to external interference and noise, that it has a high secrecy, etc.
Now the greatest point in this spread spectrum communication system is the construction of the correlator on the receiver side. At present, in the wireless spread spectrum communication system, the correlator, which is thought to be the simplest and to have a high reliability, is a device using surface acoustic wave (SAW).
In SAW convolvers there are, in general, those of correlator type (tapped delay line type) and those of convolver type. Here, although those of correlator type have a simple construction and generally a high efficiency, they are influenced seriously by the temperature coefficient of the substrate. On the contrary, those of convolver type are hardly influenced by temperature variations, but in general they have a low efficiency. However, the PN code described above of those of correlator type is fixed and they cannot deal with variations therein, while the kind of the PN code can be freely varied for those of convolver type. Consequently, if the efficiency is at a level usable in practice, the convolver type correlator can be much easily used.
Further, in the spread spectrum communication system, although the DS method can be realized in a very simple manner, because the frequency band of a high speed PN code is widened by mixing it with the base band information by means of one mixer, it has a weak point in the separation from other channels or in the remotelocal problem.
Therefore it is said that the FH/DS method is advantageous, by which the DS method is combined with the FH method, in order to overcome the weak point in the separation or the remote-local problem.
This method is a direct spreading modulation method, by which the central frequency hops periodically. FIG. 6 shows a spectrum of this modulation system. The spread spectrum signal indicated in the figure is composed of a number of spread signals. A direct spread signal covering a part of the whole band appears at each instance and on the other hand the signal as a whole has a frequency hopping pattern. The frequency hopping/direct spreading signal is used for the reasons enumerated as follows. That is, it is used for increasing the capacity of spreading the spectrum, for multiple connection and discrete address, and for multiplexing signals.
The output of the FH/DS modulator is nothing but that obtained by superposing a direct spreading modulation on a carrier hopping in frequency, as indicated in FIG. 7. In FIG. 7, reference numeral 40 is a frequency synthesizer; 41 is a code generator; 42 is a multiplier; 43 is a balanced modulator; and 44 is a transmitting antenna. The difference between this FH/DS modulation and the simple direct spreading modulation consists in that the carrier frequency is constant in the latter, while it varies in the former. Further, it is known that it is possible to construct a system, where code data coming from one code sequence generator 41 are supplied not only for determining the hopping pattern by means of the frequency synthesizer 40 but also for the balanced modulation for the direct spreading.
In the receiver, by this FH/DS method, it is a direct spreading correlator and a frequency hopping correlator superposed on each other that are used at demodulating the spread spectrum modulation before the demodulation of the base band. That is, a local reference wave is the frequency hopping/direct spread signal, which is multiplied by the input signal. FIG. 8 shows the construction of a typical FH/DS type receiver, in which reference numerals 50 and 54 are mixers; 51 is an IF filter; 52 is a base band demodulator 53 is a balanced modulator; 55 is a frequency synthesizer; 56 is a code generator; and 57 is a synchronizing circuit. The local reference oscillator is substantially the same as the modulator on the transmitter side except for the following two points, i.e. (a) that the central frequency is off-set by an amount corresponding IF and (b) that the direct spread code is not modulated by the base band input.
Although a combination of the frequency mixer 50 and the IF filter 51 is used as the correlator indicated in FIG. 8 stated above, it has a drawback that the synchronizing circuit 57 is complicated and the synchronization catch time is long.
As an FH/DS type correlator an SAW convolver has a number of practical advantages that the manner of the frequency hopping and the sort of the PN code can be freely changed. However, when the hopping frequency is selected simply arbitrarily, complexicity is produced in the timing synchronization of the hopping and improvement in the channel separation or local-distant problem cannot be obtained satisfactorily. Further, in the case where the frequency band width, which can be used, is restricted by a legal restriction, etc. the hopping method using a frequency interval of about 1/2 of the spread band width by the DS method, which has been studied heretofore, can take a small number of frequence and thus the effect of improving characteristics owing to the adoption of the FH/DS method is small.